C++ 输入输出函数

输入输出流的类结构：

标准输入输出流：
cin: 对应标准输入流 std::in;
cout:: 对应带缓冲区的std::out;
cerr: 对应不带缓冲区的 std::err， 同时在执行cerr 输出前，会先强制执行 std::cout.flush();
clog: 对应带缓冲区的 std::err;
上面说的c 对应的是character，即 character log, character err log 等；

cppreference 上有对上述几个输出输入库函数的详细定义说明;
cerr:

The global objects std::cerr and std::wcerr control output to a stream
buffer of implementation-defined type (derived from std::streambuf and
std::wstreambuf, respectively), associated with the standard C error
output stream stderr.

These objects are guaranteed to be initialized during or before the
first time an obj